Alpha-alumina flakes

ABSTRACT

The present invention relates to alumina flakes having a defined thickness and particle size distribution and to their use in varnishes, paints, automotive coatings, printing inks, masterbatches, plastics and cosmetic formulations and as substrate for effect pigments.

The present invention relates to α-Al₂O₃ flakes and to the use thereofin paints, industrial coatings, automotive coatings, printing inks,cosmetic formulations and in particular as transparent substrate foreffect pigments.

Imparting a pearlescent luster, metallic luster, color flop ormulticolor effect can be achieved by using pearlescent pigments based onnatural or synthetic transparent flakes. Pearlescent pigments based onα-Al₂O₃ flakes are well-known in the literature and commerciallyavailable under the trademark XIRALLIC® from Merck KGaA.

The important factors of a substrate for an effect pigment are theparticle size, shape, surface property, refractive index and the like.Since large and small particles have different proportions of reflectionand transmission of light on the particle surface, uniformess inparticle size is essential for a vivid and uniform color. Also theparticle size greatly affects the coloration of the pearlescent pigmentbecause it is closely related with the wavelength of the light. That is,the smaller the particle size, the larger the surface area, therebyincreasing the coloration and enhancing reflectivity, and offering amore vivid color. However, in coating metals or metal oxides on thesurface of the Al₂O₃ flakes, it is usually not easy to provide uniformcoatings on them and thus results in a decrease in the aspect ratio,which then reduces the effect of light interference thus deterioratingglossiness of the resulting pearlescent colors.

α-Al₂O₃ in the form of hexagonal flakes having a particle diametergreater than 10 μm and an aspect ratio (particle diameter/thickness) of5-10 is known from the Japanese Patent Publication No. 111239/1982.

The Japanese Patent Publication No. 72572/1991 discloses α-Al₂O₃ in theform of flakes having an average particle diameter of 0.5-3 μm.

The Japanese Patent Publication No. JP 39362/1992 describes Al₂O₃ in theform of fine platy particles of a hexagonal crystal system with theplane perpendicular to the c axis grown into a plate.

Al₂O₃ flakes composed of aluminum oxide (as a major constituent) and oftitanium dioxide (as a minor constituent) are disclosed in U.S. Pat. No.5,702,519. The Al₂O₃ flakes have an average particle diameter of about5-60 μm, a thickness less than 1 μm, and an aspect ratio of >20.

WO 2006/101306 A1 and WO 2008/026829 A1 relate to zinc doped Al₂O₃flakes and to pearlescent pigments based on these Al₂O₃ flakes. TheAl₂O₃ flakes have an average thickness of 0.5 μm or less and an averageparticle diameter of 15 μm and a large aspect ratio (diameter/thickness)of 50 μm. These zinc doped Al₂O₃ flakes are not stable under acidicconditions and thus not suitable for all applications.

The Al₂O₃ flakes of the prior art have the disadvantages that they donot have a high chemical stability and/or have not the desiredsmoothness for the use of the flakes in cosmetic and paint applications.

The object of the present invention is to provide improved Al₂O₃ flakeshaving at the same time a high chemical stability, a smooth surface andhigh whiteness.

Surprisingly, it has now been found that the properties of the aluminaflakes as such and of effect pigments based on alumina flakes can beincreased by using alumina flakes with precisely defined dimensions andparticle size and thickness distribution. Especially the opticalproperties of the alumina flakes and the effect pigments based onalumina flakes can be influenced by altering the particle sizedistribution.

Thus, the present invention relates to transparent alumina flakes whichare distinguished by the fact that they have a thickness of less than500 nm, a D₉₀-value of 30-45 μm and a D₅₀-value of 15-30 μm.

Compared to the prior art, the Al₂O₃ flakes according to the presentinvention show improved optical properties, in particular increasedchroma, higher luster, lower haze and excellent finishing and at thesame time a high chemical stability.

The alumina flakes according to the invention are used, in particular,as substrate for effect pigments, especially for the use in industrialapplications.

However, they can also be employed in all formulations where aluminaflakes are usually employed, such as, for example, in inks, coatings,preferably automotive coatings, plastics, cosmetic formulations and assubstrate for effect pigments.

The Al₂O₃ flakes of this invention have a particle size distributioncharacterized by a Gaussian distribution in which the volume sizefractions are distributed as follows:

-   -   D₅₀ is in the range of 15-30 μm, preferably 15-25 μm, including,        for example, 16-20 μm.    -   D₉₀ is in the range of 30-45 μm, preferably 30-40 μm, including,        for example, 25-36 μm and 30-35 μm.

In this patent application D₁₀, D₅₀ and D₉₀ of the alumina flakes areevaluated by using Malvern MS 2000.

The particle size distribution D₅₀ is also known as the median diameteror the medium value of the particle size distribution, it is the valueof the particle diameter at 50% in the cumulative distribution and isone of the important parameter characterizing the particle size ofpigments.

Correspondingly, the D₉₀ value indicates the maximum longitudinaldimensions of the Al₂O₃ flakes, as determined again by means of lasergranulometry in the form of sphere equivalents, which 90% of theparticles attain at maximum, or fall below, out of the entirety of allAl₂O₃ particles.

In a preferred embodiment the D₁₀ value of the alumina flakes accordingto the present invention is <9.5, preferably 9.0, including, forexample, 8.8 to 9.2.

The D₁₀ value indicates the value of the longitudinal dimension of theAl₂O₃ flakes, as determined by means of laser granulometry in the formof the sphere equivalent, which 10% of the flakes attain at most, orfall below, out of the entirety of all the Al₂O₃ flakes.

In a preferred embodiment, the Al₂O₃ flakes according to the presentinvention have a standard deviation of thickness distribution of lessthan 80 nm, preferably 5-60 nm and in particular 10-50 nm, including,for example, 20-40 nm, 25-35 nm, for example 30 nm.

In this patent application the average thickness is determined on thebasis of a cured paint film in which the Al₂O₃ flakes are orientedsubstantially plane-parallel to the substrate. For this purpose atransverse section of the cured paint film is examined under a scanningelectron microscope (SEM), the thickness of 100 Al₂O₃ flakes beingascertained and statistically averaged.

The desired size and thickness distribution can be obtained by suitableclassification of the flakes, such as by classifying through selectedscreens and the like.

The Al₂O₃ flakes according to the invention have a thickness of lessthan 500 nm, preferably 130-400 nm and in particular 150-350 nm,including, for example, 180-300 nm, 190-260 nm, and 200-250 nm.

The Al₂O₃ flakes according to the invention preferably have an aspectratio (diameter/thickness ratio) of 30-200, in particular of 50-150,including, for example, 70-120, 75-100 and 80-95.

In a preferred embodiment the Al₂O₃ flakes of the present invention areα-Al₂O₃ flakes.

The Al₂O₃ flakes can be prepared by methods known per se, as describedin the literature.

In a preferred embodiment the Al₂O₃ flakes are prepared starting from anaqueous aluminum salt solution by precipitation with an aqueous alkalicarbonate solution. An alkali metal salt like sodium or potassiumsulfate and phosphoric acid or and phosphate as and optionally at leastone dopant, for example a titanium, zirconium, silica, indium, tin, zincor indium compound are added to the starting solution. The precipitationstep is followed by drying (evaporation, dehydration by heating), andmolten salt treatment including the following steps:

-   (1) Preparation of an aqueous solution or a slurry of at least one    water-soluble and/or insoluble aluminum salt,-   (2) Adding an alkali solution to the aluminum salt solution to    precipitate aluminum hydroxide particles, and adding a phosphorous    compound and optionally at least one dopant to the aqueous solution    before, during or after the precipitation,-   (3) Evaporation of the water, followed by drying of the precipitated    product of step (2) to form the dried form of alumina containing    particle and alkali salt,-   (4) Calcination, preferably at temperatures of 900-1400° C., for    0.5-10 h, preferably 1-6 h, of the dried form obtained in step (3)    to obtain Al₂O₃ flakes in the molten salt,-   (5) Removing the water soluble part of the calcined material    obtained in step (4),-   (6) Adjust the particle size and thickness, for example by sieving,    milling and/or sedimentation.

Examples for suitable aluminum salts are aluminum sulfate, aluminumchloride, aluminum nitrate, poly aluminum chloride, aluminum hydroxide,boehmite, basic aluminum sulfate and combinations thereof.

Examples for suitable alkali metal salts which act as mineralizer,include sodium sulfate, potassium sulfate, lithium sulfate, magnesiumsulfate, sodium chloride and potassium chloride.

The phosphoric compound is preferably selected from phosphoric acid,phosphates, diphosphoric acid, sodium phosphate, ammonium phosphatedibasic and potassium phosphate. The amount of one or more phosphorouscompound(s) is preferably 0.05-2 wt. % based on the alumina flakes.

The preferred example of the pH controlling agent for the precipitationis ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate,potassium carbonate and combinations thereof.

To control the particle size, thickness, optical properties and/orsurface morphology it could be helpful to add one or more dopants inamounts of 0.01-5 wt. % based on the Al₂O₃ flake.

The dopant is preferably selected from the following group of compounds:TiO₂, ZrO₂, SiO₂, In₂O₃, SnO₂, ZnO and combinations thereof.

In a preferred embodiment the dopant is TiO₂, preferably used in amountsof 0.05-3 wt. % based on the Al₂O₃ flakes.

The Al₂O₃ flakes according to the present invention are highly suitableas substrate in the preparation of effect pigments. To this end, theyare preferably coated with at least one high refractive index layer,like a layer of metal oxide, such as, for example, TiO₂, ZrO₂, SnO₂,ZnO, Ce₂O₃, Fe₂O₃, Fe₃O₄, FeTiO₅, Cr₂O₃, CoO, Co₃O₄, VO₂, V₂O₃, NiO,furthermore of titanium suboxides (TiO₂ partially reduced with oxidationstates from <4 to 2, such as the lower oxides Ti₃O₅, Ti₂O₃, TiO),titanium oxynitrides, FeO(OH), thin semitransparent metal layers, forexample comprising Al, Fe, Cr, Ag, Au, Pt or Pd, or combinationsthereof. The TiO₂ layer may be in the rutile or anatase modification. Ingeneral, the highest quality and gloss and at the same time the moststable effect pigments are obtained when the TiO₂ is in the rutilemodification. In order to obtain the rutile modification, an additivecan be used which is able to direct the TiO₂ into the rutilemodification. Useful rutile directors are disclosed in the U.S. Pat. No.4,038,099 and U.S. Pat. No. 5,433,779 and EP 0 271 767. A preferredrutile director is SnO₂.

Preferred effect pigments based on Al₂O₃ flakes are coated with one ormore layers of metal oxides, preferably with one metal-oxide layer only,in particular with TiO₂, Fe₂O₃, Fe₃O₄, SnO₂, ZrO₂ or Cr₂O₃. Especiallypreferred are Al₂O₃ flakes coated with TiO₂ or Fe₂O₃ and mixturesthereof.

The thickness of each high-refractive-index layer depends on the desiredinterference color. The thickness of each layer on the surface of theAl₂O₃ flakes is preferably 20-400 nm, preferably 30-300 nm, inparticular 30-200 nm.

The number of layers on the surface of the Al₂O₃ flakes is preferablyone or two, furthermore three, four, five, six or seven layers.

In particular, interference packages consisting of high- andlow-refractive-index layers on the surface of the Al₂O₃ flakes result ineffect pigments having increased gloss and a further increasedinterference color or color flop.

Suitable colorless low-refractive-index materials for coating arepreferably metal oxides or the corresponding oxide hydrates, such as,for example, SiO₂, Al₂O₃, AlO(OH), B₂O₃, compounds such as MgF₂ or amixture of the said metal oxides.

In case of multilayers applied on the surface of the Al₂O₃ flakes theinterference system is, in particular, a TiO₂—SiO₂—TiO₂ layer sequence.

Furthermore, the effect pigments according to the invention may alsohave a semitransparent metal layer as outer layer. Coatings of this typeare known, for example, from DE 38 25 702 A1. The metal layers arepreferably chromium or aluminum layers having layer thicknesses of 5-25nm.

Al₂O₃ flakes can also be coated with one or more layers of a metal ormetal alloy selected e.g. from chromium, nickel, silver, bismuth,copper, tin, or hastalloy. Al₂O₃ flakes coated with a metal sulfide arecoated with sulfides e.g. of tungsten, molybdenum, cerium, lanthanum orrare earth elements.

Furthermore, the effect pigments based on Al₂O₃ flakes can be finallycoated with an organic dye as a top coat, preferably with Prussian Blueor Carmine Red.

Particularly preferred effect pigments based on the Al₂O₃ flakesaccording to the invention have the following layer sequence(s):

Al₂O₃ flake+TiO₂Al₂O₃ flake+TiO₂/Fe₂O₃Al₂O₃ flake+Fe₂O₃Al₂O₃ flake+TiO₂+Fe₂O₃Al₂O₃ flake+TiO₂+Fe₃O₄Al₂O₃ flake+TiO₂+SiO₂+TiO₂Al₂O₃ flake+Fe₂O₃+SiO₂+TiO₂Al₂O₃ flake+TiO₂/Fe₂O₃+SiO₂+TiO₂Al₂O₃ flake+TiO₂+SiO₂+TiO₂/Fe₂O₃Al₂O₃ flake+TiO₂+SiO₂Al₂O₃ flake+TiO₂+SiO₂/Al₂O₃Al₂O₃ flake+TiO₂+Al₂O₃Al₂O₃ flake+SnO₂Al₂O₃ flake+SnO₂+TiO₂Al₂O₃ flake+SnO₂+Fe₂O₃Al₂O₃ flake+SiO₂Al₂O₃ flake+SiO₂+TiO₂Al₂O₃ flake+SiO₂+TiO₂/Fe₂O₃Al₂O₃ flake+SiO₂+Fe₂O₃Al₂O₃ flake+SiO₂+TiO₂+Fe₂O₃Al₂O₃ flake+SiO₂+TiO₂+Fe₃O₄Al₂O₃ flake+SiO₂+TiO₂+SiO₂+TiO₂Al₂O₃ flake+SiO₂+Fe₂O₃+SiO₂+TiO₂Al₂O₃ flake+SiO₂+TiO₂/Fe₂O₃+SiO₂+TiO₂Al₂O₃ flake+SiO₂+TiO₂+SiO₂+TiO₂/Fe₂O₃Al₂O₃ flake+SiO₂+TiO₂+SiO₂Al₂O₃ flake+SiO₂+TiO₂+SiO₂/Al₂O₃Al₂O₃ flake+SiO₂+TiO₂+Al₂O₃Al₂O₃ flake+TiO₂+Prussian BlueAl₂O₃ flake+TiO₂+Carmine Red

The TiO₂ layer(s) in the preferred embodiments mentioned above can be inthe rutile or anatase modification. The Al₂O₃ flakes mentioned above inthe preferred embodiments can be doped or undoped.

In this application, the term “coating” or “layer” is taken to mean thecomplete enveloping of the Al₂O₃ flakes according to the invention.

The effect pigments based on doped or undoped Al₂O₃ flakes preferablyconsist of 40-90 wt. % of Al₂O₃ flakes and 10-60 wt. % of the coatingbased on the total pigment.

The Al₂O₃ flakes can be coated by wet chemical coating, by CVD or PVDprocesses.

The coating of the α-Al₂O₃ flakes with one or more layers, preferablyone or more metal oxide layers, is preferably carried out bywet-chemical methods, it being possible to use the wet-chemical coatingmethods developed for the preparation of pearlescent pigments. Methodsof this type are described, for example, in DE 14 67 468, DE 19 59 988,DE 20 09 566, DE 22 14 545, DE 22 15 191, DE 22 44 298, DE 23 13 331, DE15 22 572, DE 31 37 808, DE 31 37 809, DE 31 51 343, DE 31 51 354, DE 3151 355, DE 32 11 602, DE 32 35 017 or also in further patent documentsand other publications known to the person skilled in the art.

In the case of wet coating, the Al₂O₃ flakes are suspended in water, andone or more hydrolysable metal salts are added at a pH which is suitablefor hydrolysis, which is selected in such a way that the metal oxides ormetal-oxide hydrates are precipitated directly onto the flakes withoutsecondary precipitations occurring. The pH is usually kept constant bysimultaneous metered addition of a base and/or acid. The pigments aresubsequently separated off, washed and dried at 50-150° C. for 6-18 hand calcined for 0.5-3 h, where the calcination temperature can beoptimised with respect to the respective coating present. In general,the calcination temperatures are 500-1000° C., preferably 600-900° C. Ifdesired, the pigments can be separated off after application ofindividual coatings, dried and optionally calcined and then re-suspendedagain for the application of the further layers.

The application of a SiO₂ layer to the Al₂O₃ flake and/or to the alreadycoated Al₂O₃ flake is generally carried out by addition of a potassiumor sodium water-glass solution at a suitable pH.

Furthermore, the coating can also be carried out in a fluidised-bedreactor by gas-phase coating, it being possible to use, for example, themethods proposed in EP 0 045 851 and EP 0 106 235 for the preparation ofpearlescent pigments correspondingly.

The hue and chroma of the effect pigment based on Al₂O₃ flakes accordingto the invention can be varied in very broad limits through thedifferent choice of the coating amounts or the layer thicknessesresulting therefrom. Fine tuning for a certain hue and or chroma can beachieved beyond the pure choice of amount by approaching the desiredcolor under visual or measurement technology control.

In order to increase the light, water and weather stability, it isfrequently advisable, depending on the area of application, to subjectthe finished pigment to post-coating or post-treatment. Suitablepost-coatings or post-treatments are, for example, the processesdescribed in 22 15 191 C2, DE-A 31 51 354, DE-A 32 35 017 or DE-A 33 34598. This post-coating further increases the chemical and photochemicalstability or simplifies the handling of the pigment, in particular theincorporation into various media. In order to improve theweatherabiltiy, dispersibility and/or compatibility with the user media,it is possible, for example, for functional coatings of Al₂O₃ or ZrO₂ ormixtures thereof to be applied to the pigment surface. Furthermore,organic post-coatings are possible, for example with silanes, asdescribed, for example, in EP 0090259, EP 0 634 459, WO 99/57204, WO96/32446, WO 99/57204, U.S. Pat. No. 5,759,255, U.S. Pat. No. 5,571,851,WO 01/92425 or in J. J. Ponjeé, Philips Technical Review, Vol. 44, No.3, 81 ff. and P. H. Harding J. C. Berg, J. Adhesion Sci. Technol. Vol.11 No. 4, pp. 471-493.

In accordance with the present invention, an effect pigment based onAl₂O₃ flakes having the desired size distribution has been found usefulin all types of compositions, including plastics, cosmetics, and inparticular in automotive paints.

The Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes accordingto the invention are compatible with a multiplicity of color systems,preferably from the area of paints, automotive coatings, industrialcoatings, and printing inks and cosmetic formulations. For thepreparation of printing inks for, for example, gravure printing,flexographic printing, offset printing and offset overvarnishing, amultiplicity of binders, in particular water-soluble grades, as sold,for example, by BASF, Marabu, Pröll, Sericol, Hartmann, Gebr. Schmidt,Sicpa, Aarberg, Siegberg, GSB-Wahl, Follmann, Ruco or Coates Screen INKSGmbH, is suitable. The printing inks can be water-based orsolvent-based. The pigments are furthermore also suitable for the lasermarking of paper and plastics and for applications in the agriculturalsector, for example for greenhouse sheeting, and, for example, for thecoloring of tent awnings.

It goes without saying that, for the various applications, the coatedand uncoated Al₂O₃ flakes according to the present invention can alsoadvantageously be used in blends with organic dyes, organic pigments orother pigments, such as, for example, transparent and opaque white,colored and black pigments, and with flake-form iron oxides, holographicpigments, LCPs (liquid crystal polymers) and conventional transparent,colored and black luster pigments based on metal oxide-coated mica andSiO₂ flakes, etc. The Al₂O₃ flakes and the effect pigments based onAl₂O₃ flakes according to the invention can be mixed in any ratio withcommercially available pigments and fillers.

Fillers which may be mentioned are, for example, natural and syntheticmica, nylon powder, pure or filled melamine resins, talc, SiO₂, glasses,kaolin, oxides or hydroxides of aluminum, magnesium, calcium or zinc,BiOCl, barium sulfate, calcium sulfate, calcium carbonate, magnesiumcarbonate, carbon, and physical or chemical combinations of thesesubstances. There are no restrictions regarding the particle shape ofthe filler. It can be, for example, flake-form, spherical orneedle-shaped in accordance with requirements.

The Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes accordingto the invention are simple and easy to handle. The Al₂O₃ flakes and theeffect pigments based on Al₂O₃ flakes can be incorporated into thesystem in which it is used by simple stirring. Laborious milling anddispersing of the Al₂O₃ flakes and the effect pigments is not necessary.

The Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes accordingto the invention can be used for pigmenting coating materials, printinginks, plastics, agricultural films, button pastes, for the coating ofseed, for the coloring of food, coatings of medicaments or cosmeticformulations. The concentration of the Al₂O₃ flakes and the effectpigments in the system in which it is to be used for pigmenting isgenerally between 0.01 and 50% by weight, preferably between 0.1 and 5%by weight, based on the overall solids content of the system. Thisconcentration is generally dependent on the specific application.

Plastics containing the Al₂O₃ flakes and the effect pigments based onAl₂O₃ flakes according to the invention in amounts of 0.1 to 50% byweight, in particular from 0.5 to 7% by weight, are frequently notablefor a particular gloss effect.

In the coating sector, especially in automotive coatings and automotivefinishing, the effect pigments based on Al₂O₃ flakes according to theinvention are employed in amounts of 0.5-10% by weight.

In the coating material, the Al₂O₃ flakes and the effect pigments basedon Al₂O₃ flakes according to the invention have the advantage that thedesired color and gloss is obtained by a single-layer coating (one-coatsystems or as a base coat in a two-coat system).

In the pigmentation of binder systems, for example for paints andprinting inks for intaglio, offset or screen printing, the effectpigments based on Al₂O₃ flakes with STAPe-aluminum and gold bronzepastes from Eckart GmbH have proven particularly suitable. The effectpigment is incorporated into the printing ink in amounts of 2-50% byweight, preferably 5-30% by weight and, in particular, 8-15% by weight.The printing inks containing the effect pigment according to theinvention in combination with a metal effect pigment exhibits purer huesand is of improved printability owing to the good viscosity values.

The invention likewise provides pigment preparations containing coatedor uncoated Al₂O₃ flakes according to the present invention and furthereffect pigments, binders and, if desired, additives, the saidpreparations being in the form of substantially solvent-free,free-flowing granules. Such granules contain up to 95% by weight of theeffect pigment according to the invention. A pigment preparation inwhich the Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakesaccording to the invention is pasted up with a binder and with waterand/or an organic solvent, with or without additives, and the paste issubsequently dried and brought into a compact particulate form, e.g.granules, pellets, briquettes, a masterbatch or tablets, is particularlysuitable as a precursor for printing inks.

The invention thus also relates to the use of the coated (=effectpigments) or uncoated Al₂O₃ flakes in formulations from the areas ofpaints, coatings, automobile coatings, automotive finishing, industrialcoatings, paints, powder coatings, printing inks, security printinginks, plastics, ceramic materials, cosmetics. The coated and uncoatedAl₂O₃ flakes can furthermore be employed in glasses, in paper, in papercoating, in toners for electrophotographic printing processes, in seed,in greenhouse sheeting and tarpaulins, in thermally conductive,self-supporting, electrically insulating, flexible sheets for theinsulation of machines or devices, as absorber in the laser marking ofpaper and plastics, as absorber in the laser welding of plastics, inpigment pastes with water, organic and/or aqueous solvents, in pigmentpreparations and dry preparations, such as, for example, granules, forexample in clear coats in the industrial and automobile sectors, insunscreens, as filler, in particular in automobile coatings andautomotive finishing.

All percentage data in this application are percent by weight, unlessindicated otherwise.

The following examples are intended to explain the invention in greaterdetail, but without restricting it. Above and below, all percentages arepercent by weight.

EXAMPLES Comparative Example 1 Example 2 of U.S. Pat. No. 5,702,519

111.9 g of aluminum sulfate 18-hydrate, 57.3 g of anhydrous sodiumsulfate and 46.9 g of potassium sulfate are dissolved in 300 ml ofdeionized water by heating above 60° C. 3.0 g of 34.4% solution oftitanyl sulfate solution is added to this solution. The resultingsolution is designated as the aqueous solution (a).

0.45 g of sodium tertiary phosphate 12-hydrate and 55.0 g of sodiumcarbonate are added to 150 l of deionized water. The resulting solutionis designated as the aqueous solution (b).

The aqueous solution (b) is added with stirring to the aqueous solution(a) kept at about 60° C. Stirring is continued for further 15 minutes.The resulting mixture of the two solutions (a) and (b) is a gel. Thisgel is evaporated to dryness, and the dried product is heated at 1200°C. for 5 hours. Water is added to the heated product to dissolve freesulfate. Insoluble solids are filtered off, washed with water, andfinally dried. The obtained alumina flake is examined by X-raydiffractometry. The diffraction pattern has only peaks attributed tocorundum structure (α-alumina structure). D₅₀ is 13.0 μm and D₉₀ is 22.0μm and the thickness is 200 nm.

The standard deviation of the thickness distribution of the α-Al₂O₃flake is 83 nm.

Example 1 Production of Al₂O₃ Flakes

74.6 g of aluminum sulfate 18-hydrate, 57.1 g of poly aluminum chloride(PAC: Central Glass Co., LTD, 10% solution as Al₂O₃), 57.3 g ofanhydrous sodium sulfate, and 46.9 g of potassium sulfate are dissolvedin 300 ml of deionized water by heating above 60° C. 3.0 g of 34.4% of atitanyl sulfate solution are added to the solution. The resultingsolution is designated as the aqueous solution (a).

0.45 g of sodium tertiary phosphate 12-hydrate and 55.0 g of sodiumcarbonate are added to 300 ml of deionized water. The resulting solutionis designated as the aqueous solution (b).

The aqueous solution (b) is added with stirring to the aqueous solution(a) and kept at about 60° C. Stirring is continued for 1 h. The obtainedmixture of solution (a) and solution (b) is a slurry. This slurry isevaporated to dryness and the dried product is heated at 1150° C. for 6h. Water is added to the heated product to dissolve free sulfate.Insoluble solids are filtered off and washed with water. Finally, theproduct is dried.

The obtained alumina flake is examined by X-ray diffractometry. Thediffraction pattern have only peaks attributed to corundum structure(α-alumina structure).

The obtained Al₂O₃ flakes have a D₅₀ value of 16.0 μm and D₉₀ value of30.8 μm and a thickness of 200 nm.

The standard deviation of the thickness distribution of the α-Al₂O₃flake is 28 nm.

Example 2 Production of Al₂O₃ Flakes

74.6 g of aluminum sulfate 18-hydrate, 57.1 g of poly aluminum chloride(PAC: Central Glass Co., LTD, 10% solution as Al₂O₃), 57.3 g ofanhydrous sodium sulfate, and 46.9 g of potassium sulfate are dissolvedin 300 ml of deionized water by heating above 60° C. 3.0 g of 34.4% of atitanyl sulfate solution and 5.5 g of 5.0% indium chloride(III) solutionare added to the solution. The resulting solution is designated as theaqueous solution (a).

0.45 g of sodium tertiary phosphate 12-hydrate and 55.0 g of sodiumcarbonate are added to 300 ml of deionized water. The resulting solutionis designated as the aqueous solution (b).

The aqueous solution (b) is added with stirring to the aqueous solution(a) and kept at about 60° C. Stirring is continued for 1 h. The obtainedmixture of solution (a) and solution (b) is a slurry. This slurry isevaporated to dryness and the dried product is heated at 1200° C. for 4h. Water is added to the heated product to dissolve free sulfate.Insoluble solids are filtered off and washed with water. Finally, theproduct is dried.

The obtained alumina flake is examined by X-ray diffractometry. Thediffraction pattern have only peaks attributed to corundum structureα-alumina structure).

The obtained Al₂O₃ flakes have a D₅₀ value of 19.0 μm and D₉₀ value of35.6 μm and a thickness of 250 nm.

The standard deviation of the thickness distribution of the α-Al₂O₃flake is 32 nm.

Comparative Example 1.1 Coating of Al₂O₃ Flakes

20 g alumina flakes of Comparative Example 1 are suspended in 400 ml ofdeionized water. To the resulting suspension (kept at about 65° C.) isadded a solution containing 125 g of TiCl₄ per liter. Simultaneously a10% solution of NaOH is added to keep the pH at 2.1. The addition of theTiCl₄ solution is stopped when the resulting product takes on a silverycolor. The suspending solids are filtered off, washed with water, anddried. Finally, the dried solids are calcined at 850° C. for 30 minutesto give a whitish and a little glossy pearlescent pigment.

Example 1.1 Coating of Al₂O₃ Flakes

20 g alumina flakes of Example 1 are suspended in 400 ml of deionizedwater. To the resulting suspension (kept at about 65° C.) is added asolution containing 125 g of TiCl₄ per liter. Simultaneously a 10%solution of NaOH is added to keep the pH at 2.1. The addition of theTiCl₄ solution is stopped when the resulting product takes on a silverycolor. The suspending solids are filtered off, washed with water, anddried. Finally, the dried solids are calcined at 850° C. for 30 minutesto give a highly whitish and glossy pearlescent pigment. At the lusterangle, the glossy appearance can be seen at a more wider angle comparedto Comparative Example 1.1.

Example 2.1 Coating of Al₂O₃ Flakes

20 g alumina flakes of Example 2 are suspended in 400 ml of deionizedwater. To the resulting suspension (kept at about 65° C.) is added asolution containing 125 g of TiCl₄ per liter. Simultaneously a 10%solution of NaOH is added to keep the pH at 2.1. The addition of theTiCl₄ solution is stopped when the resulting product takes on a silverycolor. The suspending solids are filtered off, washed with water, anddried. Finally, the dried solids are calcined at 850° C. for 30 minutesto give a highly whitish and glossy pearlescent pigment. At the lusterangle, the glossy appearance can be seen at a more wider angle comparedto Comparative Example 1.1.

Comparative Example 1.2 Coating of Al₂O₃ Flakes

20 g alumina flakes of Comparative Example 1 are suspended in 400 ml ofdeionized water. To the resulting suspension (kept at about 75° C.) isadded a solution containing 300 g of FeCl₃ per liter. Simultaneously a10% solution of NaOH was added to keep the pH at 3.0. The addition ofthe FeCl₃ solution is stopped when the resulting product takes on a mostreddish color. The suspending solids are filtered off, washed withwater, and dried. Finally, the dried solids are calcined at 800° C. for30 minutes to give a moderate luster and brownish red coloredpearlescent pigment.

Example 1.2

20 g of the alumina flakes of Example 1 are suspended in 400 ml ofdeionized water. To the resulting suspension (kept at about 75° C.) isadded a solution containing 300 g of FeCl₃ per liter. Simultaneously a10% solution of NaOH was added to keep the pH at 3.0. The addition ofthe FeCl₃ solution is stopped when the resulting product takes on a mostreddish color. The suspending solids are filtered off, washed withwater, and dried. Finally, the dried solids are calcined at 800° C. for30 minutes to give a high luster and pure red colored pearlescentpigment. The pigment shows a higher luster and more cleaner red colorcompared to the appearance of the pigment of Comparative Example 1.2.

Comparative Example 1.3 Coating of Al₂O₃ Flakes

20 g alumina flakes of Comparative Example 1 are suspended in 400 ml ofdeionized water. To the resulting suspension (kept at about 65° C.) isadded a solution containing 50 g of SnCl₄ per liter. Simultaneously a10% solution of NaOH is added to keep the pH at 2.1 until the totaladding volume of SnCl₄ solution become 17 ml. Next the solutioncontaining 125 g of TiCl₄ per liter was added to the resultingsuspension. Simultaneously a 10% solution of NaOH is added to keep thepH at 2.1. The addition of the TiCl₄ solution is stopped when theresulting product takes on a silvery color. The suspending solids arefiltered off, washed with water, and dried. Finally, the dried solidsare calcined at 850° C. for 30 minutes to give a whitish and a littleglossy pearlescent pigment.

Example 1.3 Coating of Al₂O₃ Flakes

20 g alumina flakes of Example 1 are suspended in 400 ml of deionizedwater. To the resulting suspension (kept at about 65° C.) is added asolution containing 50 g of SnCl₄ per liter. Simultaneously a 10%solution of NaOH is added to keep the pH at 2.1 until the total addingvolume of SnCl₄ solution become 17 ml. Next the solution containing 125g of TiCl₄ per liter was added to the resulting suspension.Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1.The addition of the TiCl₄ solution is stopped when the resulting producttakes on a silvery color. The suspending solids are filtered off, washedwith water, and dried. Finally, the dried solids are calcined at 850° C.for 30 minutes to give a highly whitish and highly glossy pearlescentpigment. At the luster angle, the glossy appearance can be seen at amore wider angle compared to Comparative Example 1.3

Comparative Example 1.4 Coating of Al₂O₃ Flakes

20 g alumina flakes of Comparative Example 1 are suspended in 400 ml ofdeionized water. To the resulting suspension (kept at about 65° C.) isadded a solution containing 125 g of TiCl₄ per liter. Simultaneously a10% solution of NaOH is added to keep the pH at 2.1. The addition of theTiCl₄ solution is stopped when the resulting product takes on ayellowish color. Then a solution containing 50 g of Na₂SiO₃ per liter isadded to the resulted suspension. Simultaneously a 10% solution of HClis added to keep the pH at 7. Next a solution containing 125 g of TiCl₄per liter is added to the resulted suspension. Simultaneously a 10%solution of NaOH is added to keep the pH at 2.1. The addition of theTiCl₄ solution is stopped when the resulting product takes on a bluishcolor. The suspending solids are filtered off, washed with water, anddried. Finally, the dried solids are calcined at 850° C. for 30 minutesto give a blue-whitish and a little glossy pearlescent pigment.

Example 1.4 Coating of Al₂O₃ Flakes

20 g alumina flakes of Example 1 are suspended in 400 ml of deionizedwater. To the resulting suspension (kept at about 65° C.) is added asolution containing 125 g of TiCl₄ per liter. Simultaneously a 10%solution of NaOH is added to keep the pH at 2.1. The addition of theTiCl₄ solution is stopped when the resulting product takes on ayellowish color. Then a solution containing 50 g of Na₂SiO₃ per liter isadded to the resulted suspension. Simultaneously a 10% solution of HClis added to keep the pH at 7. Next a solution containing 125 g of TiCl₄per liter is added to the resulted suspension. Simultaneously a 10%solution of NaOH is added to keep the pH at 2.1. The addition of theTiCl₄ solution is stopped when the resulting product takes on a bluishcolor. The suspending solids are filtered off, washed with water, anddried. Finally, the dried solids are calcined at 850° C. for 30 minutesto give a highly bluish and glossy pearlescent pigment. At the lusterangle, the stronger bluish color and higher glossy appearance can beseen at a more wider angle compared to Comparative Example 1.4.

Measurements Evaluation for Particle Size D₁₀₅₀ and D₉₀

D₁₀, D₅₀ and D₉₀ of the alumina flakes are evaluated by using MalvernMS2000.

Determination of the Thickness and Particle Size and the ThicknessDistribution

0.01 g/l of the alumina flake slurry is prepared and 0.1 ml of thisslurry is dropped onto a flat substrate like a silicon wafer. Thesubstrate is dried and cut to adequate size. The substrate is set withalmost vertically tilted angle on the base of SEM (Scanning electronicmicroscope) and the thickness of the alumina flake is determined.

The thickness of more than 100 alumina flakes is measured for thecalculation of the thickness distribution. The standard deviation of thethickness is calculated with the Gaussian distribution equation.

Preparation for a Sprayed Panel

A base coat paint for automobiles is prepared according to the followingformulation.

<Base coat system> Acrylic-melamine resin system “ACRYDIC ® 47-712”* 70pbw “SUPERBEKKAMINE ® G821-60”** 30 pbw Toluene 30 pbw Ethyl acetate 50pbw n-Butanol 110 pbw SOLVESSO ®#150*** 40 pbw *Acrylic resin fromDainippon Ink & Chemicals, Inc. **Melamine resin from Dainippon Ink &Chemicals, Inc. ***Naphtha (petroleum)

The above acrylic-melamine resin system (100 pbw) is incorporated with20 pbw of the pearlescent pigment according to Example 1 or 2. Theresulting compound is diluted with a thinner so that the resulting painthas an adequate consistency for spraying (12-15 seconds, for cup #4).This paint is applied to a substrate by spraying to form a base coatlayer.

The base coated layer is coated further with a colorless top clear coatpaint, which is prepared according to the following formulation.

<Top clear coat system> “ACRYDIC ® 47-712” 14 pbw “SUPERBEKKAMINE ®L117-60” 6 pbw Toluene 4 pbw MIBK 4 pbw Butyl cellosolve 3 pbw

The top clear coating is exposed to air at 40° C. for 30 minutes andthen cured at 135° C. for 30 minutes.

Haze-gloss (BYK) is the analyzing equipment to evaluate luster and haze.In this patent application, the value of mirror gloss on 60° is measuredby Haze-gloss and represents the value of the luster. Compared to theprior art the coated Al₂O₃ flakes show very high values for the luster.High luster values are necessary to achieve a good appearance in theapplications. The haze values measured by this equipment are affected byspreading of the reflection angle. In this patent application, the widerspreading angle is important for the pearlescent appearance. The coatedAl₂O₃ flakes according to the present invention show very highhaze-gloss values.

To differentiate the evaluation result, panels are used which aresprayed with a base coat before they are coated with a top clear coatfor the haze-gloss measurement.

Wave-scan dual (BYK) is used as the analyzing equipment to measure thesurface flatness of the samples. Wa value represents the cyclic flatnessin the range of 0.1-0.3 mm. A smaller value in this patent applicationrepresents a flatter surface showing the advantages of the pigmentsaccording to the present invention.

Sprayed panels with a top clear coat are measured for Wa. Flattersurfaces have better finishing appearance.

The optical properties of the pearlescent pigments according to theabove given examples are summarized in the following table:

TABLE 1 Standard Optical Particle size deviation properties TiO₂distribution of the of TiO₂ coated coated of the Al₂O₃ Thickness Al₂O₃flakes Al₂O₃ Al₂O₃ flakes (μm) distribution haze- flakes flakes D₁₀ D₅₀D₉₀ (nm) luster gloss Wa E1 E1.1 9.0 16.0 30.8 28 40 21 9 E2 E2.1 9.119.0 35.6 32 46 25 12 CE1 CE1.1 4.8 13.0 22.0 83 11 4 21

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding European application No. EP13002293.2, filed Apr. 30, 2013 are incorporated by reference herein.

1. Al₂O₃ flakes having a particle thickness of less than 500 nm and aD₅₀-value of 15-30 μm and a D₉₀-value of 30-45 μm.
 2. Al₂O₃ flakesaccording to claim 1, which are α-alumina flakes.
 3. Al₂O₃ flakesaccording to claim 1, which have a standard deviation of thicknessdistribution of less than 80 nm.
 4. Al₂O₃ flakes according to claim 1,which are doped with TiO₂, ZrO₂, SiO₂, SnO₂, In₂O₃, or ZnO or acombination thereof.
 5. Al₂O₃ flakes according to claim 4, wherein theamount of doping is 0.01-5% by weight based on the Al₂O₃ flake.
 6. Al₂O₃flakes according to claim 4, which are doped with TiO₂.
 7. Al₂O₃ flakesaccording to claim 1, which are coated with at least one layer of ametal oxide, a mixture of at least two metal oxides, a metal, a metalsulphide, a titanium suboxide, a titanium oxynitride, FeO(OH), SiO₂, ametal alloy or a rare earth compound.
 8. Al₂O₃ flakes according to claim1, which are coated with at least one layer of a metal oxide or amixture of at least two metal oxides.
 9. Al₂O₃ flakes according to claim1, which are coated with the following layer sequence: Al₂O₃ flake+TiO₂Al₂O₃ flake+TiO₂/Fe₂O₃ Al₂O₃ flake+Fe₂O₃ Al₂O₃ flake+TiO₂+Fe₂O₃ Al₂O₃flake+TiO₂+Fe₃O₄ Al₂O₃ flake+TiO₂+SiO₂+TiO₂ Al₂O₃ flake+Fe₂O₃+SiO₂+TiO₂Al₂O₃ flake+TiO₂/Fe₂O₃+SiO₂+TiO₂ Al₂O₃ flake+TiO₂+SiO₂+TiO₂/Fe₂O₃ Al₂O₃flake+TiO₂+SiO₂ Al₂O₃ flake+TiO₂+SiO₂/Al₂O₃ Al₂O₃ flake+TiO₂+Al₂O₃ Al₂O₃flake+SnO₂ Al₂O₃ flake+SnO₂+TiO₂ Al₂O₃ flake+SnO₂+Fe₂O₃ Al₂O₃ flake+SiO₂Al₂O₃ flake+SiO₂+TiO₂ Al₂O₃ flake+SiO₂+TiO₂/Fe₂O₃ Al₂O₃ flake+SiO₂+Fe₂O₃Al₂O₃ flake+SiO₂+TiO₂+Fe₂O₃ Al₂O₃ flake+SiO₂+TiO₂+Fe₃O₄ Al₂O₃flake+SiO₂+TiO₂+SiO₂+TiO₂ Al₂O₃ flake+SiO₂+Fe₂O₃+SiO₂+TiO₂ Al₂O₃flake+SiO₂+TiO₂/Fe₂O₃+SiO₂+TiO₂ Al₂O₃ flake+SiO₂+TiO₂+SiO₂+TiO₂/Fe₂O₃Al₂O₃ flake+SiO₂+TiO₂+SiO₂ Al₂O₃ flake+SiO₂+TiO₂+SiO₂/Al₂O₃ Al₂O₃flake+SiO₂+TiO₂+Al₂O₃ Al₂O₃ flake+TiO₂+Prussian Blue or Al₂O₃flake+TiO₂+Carmine Red.
 10. Al₂O₃ flakes according to claim 1, which arecoated with TiO₂ in the rutile or anatase modification.
 11. Al₂O₃ flakesaccording to claim 1, which are coated with TiO₂ in the rutilemodification.
 12. Al₂O₃ flakes according to claim 1, which consist of40-90 wt. % of Al₂O₃ flakes and 10-60 wt. % of one or more coatingsbased on the total weight of the Al₂O₃ flakes plus and the one or morecoatings.
 13. A process for preparing Al₂O₃ flakes according to claim 1,comprising (1) preparing an aqueous solution or a slurry of at least onewater-soluble and/or insoluble aluminum salt, (2) adding an alkalisolution to the aluminum salt solution to precipitate aluminum hydroxideparticles, and adding a phosphorous compound and optionally at least onedopant to the aqueous solution before, during or after theprecipitation, (3) evaporating the water, followed by drying of theprecipitated product obtained in (2) to form a dried form of aluminacontaining particle and alkali salt, (4) calcining the dried formobtained in (3) to obtain Al₂O₃ flakes in a molten salt, (5) removingthe water soluble part of the calcined material obtained in (4), and (6)adjusting the particle size and thickness.
 14. A product selected fromthe group consisting of base material for effect pigments, paints,coatings, automobile coatings, automotive finishing, industrialcoatings, powder coatings, printing inks, security printing inks,plastics, ceramic materials, cosmetics, glasses, paper, paper coating,toners for electrophotographic printing processes, seeds, greenhousesheeting, tarpaulins, thermally conductive self-supporting electricallyinsulating flexible sheets for the insulation of machines or devices,absorber in the laser marking of paper and plastics, absorber in thelaser welding of plastics, pigment pastes with water, organic solvents,aqueous solvents, pigment preparations and dry preparations, comprisingAl₂O₃ flakes according to claim
 1. 15. A composition containing coatedor uncoated Al₂O₃ flakes according to claim 1 in an amount of 0.01-95%by weight, based on the composition as a whole, and at least one furtheringredient forming said composition.
 16. A composition containing Al₂O₃flakes according to claim 1, and at least one component selected fromthe group consisting of water, polyols, polar oils, nonpolar oils, fats,waxes, film formers, polymers, copolymers, surfactants, free-radicalscavengers, antioxidants, stabilisers, odour enhancers, silicone oils,emulsifiers, solvents, preservatives, thickeners, rheological additives,fragrances, colorants, effect pigments, UV absorbers, surface-activeassistants, cosmetic active compounds, fillers, binders, pearlescentpigments, color pigments and organic dyes.